== CFG Participating Investigators contributing to the understanding of this paradigm ==

== CFG Participating Investigators contributing to the understanding of this paradigm ==

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== Progress toward understanding this GBP paradigm ==

== Progress toward understanding this GBP paradigm ==

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This section documents what is currently known about MAG, its carbohydrate ligand(s), and how they interact to mediate cell communication. Further information can be found in the GBP Molecule Pages for [http://www.functionalglycomics.org/glycomics/molecule/jsp/viewGbpMolecule.jsp?gbpId=cbp_hum_Itlect_271&sideMenu=no human] and [http://www.functionalglycomics.org/glycomics/molecule/jsp/viewGbpMolecule.jsp?gbpId=cbp_mou_Itlect_196&sideMenu=no mouse] MAG (a.k.a. Siglec-4a) in the CFG database.

MAG (Siglec-4) is expressed exclusively on myelin, which is produced by oligodendrocytes (the myelinating cells of the central nervous system) and Schwann cells (the myelinating cells of the peripheral nervous system). In both central and peripheral nervous systems, MAG is enriched on the innermost wrap of myelin, directly apposing the axon surface.<ref name="quarles2007">Quarles RH. Myelin-associated glycoprotein (MAG): past, present and beyond. J Neurochem. 100, 1431-1448 (2007).</ref><br> MAG recognizes as ligands sialoside sequences found on gangliosides that are abundant in axonal membranes<ref name="Schnaar 2009"/>.

<br>Siglec-4 is a heavily glycosylated protein of about 100kDa with 30% of its mass being made up by carbohydrates distributed over eight glycosylation sites. The extracellular part of Siglec-4 consists of five Ig-like domains (one V-set domain and four C2-set domains). Two splice variants for Siglec-4 are found in mammals, L-MAG (72kDa) and S-MAG (67kDa), which differ in their cytoplasmic domain. L-MAG contains a tyrosine phosphorylation site<ref name="umemori1994">Umemori, H., Sato, S., Yagi, T., Aizawa, S., Yamamoto, T. Initial events of myelination involve Fyn tyrosine kinase signalling. Nature 367, 572-576 (1994)</ref><ref name="jaramillo1994">Jaramillo, M. L., Afar, D. E., Almazan, G., Bell, J. C. Identification of tyrosine 620 as the major phosphorylation site of myelin-associated glycoprotein and its implication in interacting with signaling molecules. J Biol Chem. 269, 27240-27245 (1994)</ref> that is missing in S-MAG.

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=== Biological roles of GBP-ligand interaction ===

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=== Biological roles of GBP-ligand interaction ===

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MAG is expressed on the innermost myelin membrane wrap, directly apposed to the axon surface. Although it is not required for myelination, MAG enhances long-term axon survival, helps structure myelin gaps (nodes of Ranvier) essential for rapid nerve conduction, regulates the axon cytoskeleton and protects axons from acute toxic insults. In addition to its role in axon-myelin stabilization, MAG inhibits axon regeneration after injury; MAG on residual myelin membranes at injury sites actively signals axons to halt elongation. Whether MAG's stabilizing effects and its inhibition of axon regeneration are part of the same signaling system is under investigation.

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MAG has multiple receptors on the axon surface, including gangliosides GD1a/GT1b, the GPI-anchored Nogo receptors (NgR1 and NgR2), and transmembrane proteins PirB and β-integrin. Some of these interactions involve MAG's glycan binding capability, while others may not. The following biological roles of MAG have been experimentally linked to its glycan binding activity using genetic, biochemical, and/or pharmacological criteria:

The best examples of CFG contributions to this paradigm are described below, with links to specific data sets. For a complete list of CFG data and resources relating to this paradigm, see the [http://www.functionalglycomics.org/glycomics/search/jsp/landing.jsp?query=Siglec-4&maxresults=20 CFG database search results for Siglec-4].

The best examples of CFG contributions to this paradigm are described below, with links to specific data sets. For a complete list of CFG data and resources relating to this paradigm, see the [http://www.functionalglycomics.org/glycomics/search/jsp/landing.jsp?query=Siglec-4&maxresults=20 CFG database search results for Siglec-4].

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=== Glycogene microarray ===

=== Glycogene microarray ===

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Probes for mouse and human MAG (under the name Siglec-4) have been included on all four versions of the CFG glycogene microarray.

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=== Knockout mouse lines ===

=== Knockout mouse lines ===

The CFG has [https://www.functionalglycomics.org/glycomics/publicdata/phenotyping.jsp phenotyped] the MAG-deficient mouse.

The CFG has [https://www.functionalglycomics.org/glycomics/publicdata/phenotyping.jsp phenotyped] the MAG-deficient mouse.

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== Related GBPs ==

== Related GBPs ==

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Compared to other Siglecs, Siglec-4 is most conserved. Based on sequence similarity orthologous proteins can be identified in all vertebrate genomes available so far (several mammals, chicken, Xenopus, zebrafish and fugu). Sialic acid binding activity selective for 2,3-linked Sia has been shown for the avian ortholog (SMP<ref name="collins1996"/> and fish Siglec-4 from zebrafish (Danio rerio) and fugu (Takifugu rubripes)<ref name=" Lehmann, F.2004"> Lehmann, F., Gäthje, H., Kelm, S., Dietz, F. Evolution of sialic acid-binding proteins: molecular cloning and expression of fish siglec-4. Glycobiology 14, 959-968 (2004)</ref>). Whereas the primary sequences of the Sia-binding N-terminal domains is 97 % identical between rodents and man and share over 50 % sequence identity between fish and mammals, the cytoplasmic tail is much less conserved (20% identical amino acids between fish and mammals<ref name=" Lehmann, F.2004"></ref>).

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== References ==

== References ==

Current revision as of 23:36, 30 March 2011

Myelin-associated glycoprotein (MAG, Siglec-4) is unique among the siglecs in that it is expressed exclusively on neuronal glial cells[1][2]. It is the most highly conserved among the siglecs in mammalian species. This siglec paradigm is unique in its activity of stabilizing axon-myelin interactions. MAG has a cytoplasmic domain that is devoid of ITIMs, but contains a tyrosine-based motif associated with binding the FYN tyrosine kinase, believed to play a role in its activity in myelin-axon interactions. MAG recognizes as ligands sialoside sequences found on gangliosides that are abundant in axonal membranes[2]. It is one of several proteins in myelin that negatively regulate axon outgrowth following tissue injury, an activity that involves MAG-ligand interactions. Evidence suggests that inhibition of MAG-ligand interactions may enhance neurite outgrowth and repair of injured neurons[3][4][5].

CFG Participating Investigators contributing to the understanding of this paradigm

Several CFG Participating Investigators (PIs) have contributed to identification of MAG as a siglec and to understanding the functions of MAG, including: Paul Crocker, Sørge Kelm, James Paulson, Ronald Schnaar

Progress toward understanding this GBP paradigm

This section documents what is currently known about MAG, its carbohydrate ligand(s), and how they interact to mediate cell communication. Further information can be found in the GBP Molecule Pages for human and mouse MAG (a.k.a. Siglec-4a) in the CFG database.

Carbohydrate ligands

The glycan specificity of Siglec-4 has been investigated using resialylated erythrocytes[6], gangliosides[7][8], and inhibition assays with oligosaccharides[9][10].

Determinant recognized:

on glycolipids and/or glycoproteins

Specificity for linkage of sialic acid to underlying Gal:

about 10-fold better binding to Neu5Acα2,3Gal-R than Neu5Acα2,6Gal-R

Underlying glycan structures can enhance binding:

Similar binding to the following structures as soluble glycosides[9]. Enhanced binding to first structure in intact gangliosides[11][12].

Cellular expression of GBP and ligands

MAG (Siglec-4) is expressed exclusively on myelin, which is produced by oligodendrocytes (the myelinating cells of the central nervous system) and Schwann cells (the myelinating cells of the peripheral nervous system). In both central and peripheral nervous systems, MAG is enriched on the innermost wrap of myelin, directly apposing the axon surface.[13] MAG recognizes as ligands sialoside sequences found on gangliosides that are abundant in axonal membranes[2].

Structure

Siglec-4 is a heavily glycosylated protein of about 100kDa with 30% of its mass being made up by carbohydrates distributed over eight glycosylation sites. The extracellular part of Siglec-4 consists of five Ig-like domains (one V-set domain and four C2-set domains). Two splice variants for Siglec-4 are found in mammals, L-MAG (72kDa) and S-MAG (67kDa), which differ in their cytoplasmic domain. L-MAG contains a tyrosine phosphorylation site[15][16] that is missing in S-MAG.

Biological roles of GBP-ligand interaction

MAG is expressed on the innermost myelin membrane wrap, directly apposed to the axon surface. Although it is not required for myelination, MAG enhances long-term axon survival, helps structure myelin gaps (nodes of Ranvier) essential for rapid nerve conduction, regulates the axon cytoskeleton and protects axons from acute toxic insults. In addition to its role in axon-myelin stabilization, MAG inhibits axon regeneration after injury; MAG on residual myelin membranes at injury sites actively signals axons to halt elongation. Whether MAG's stabilizing effects and its inhibition of axon regeneration are part of the same signaling system is under investigation.
MAG has multiple receptors on the axon surface, including gangliosides GD1a/GT1b, the GPI-anchored Nogo receptors (NgR1 and NgR2), and transmembrane proteins PirB and β-integrin. Some of these interactions involve MAG's glycan binding capability, while others may not. The following biological roles of MAG have been experimentally linked to its glycan binding activity using genetic, biochemical, and/or pharmacological criteria:
1. Long term axon stabilization: B4galnt1-null mice, which lack the termini of complex gangliosides, display the same progressive axon degeneration phenotype as Mag-null mice. Double null mice (B4galnt1, Mag) are similar. [14]
2. Nodes of Ranvier: B4galnt1-null and Mag-null mice have similar deficits in the structures of Nodes of Ranvier[17][18]
3. Cytoskeletal organization: B4galnt1-null, Mag-null and double-null mice have similarly reduced neurofilament spacing and reduced axon diameter.[14]
4. Axon protection: MAG-mediated protection of axons from toxic insults is diminished in B4galnt1-null mice or after treatment of axons with sialidase.[19][20]
5. Regulating axon regeneration: MAG-mediated inhibition of axon regeneration is diminished in B4galnt1-null mice, after treatment with sialidase, or by addition of MAG-binding soluble glycans.[21][5]
MAG signaling is bidirectional,[13] into the myelinating cells and into myelin-ensheathed axons. Signaling into myelinating cells may involve tyrosine phosphorylation of the MAG intracellular domain downstream of ligand engagement,[15][16] whereas signals into the axon are likely to involve activation of the small non-receptor GTPase RhoA.[22]

CFG resources used in investigations

The best examples of CFG contributions to this paradigm are described below, with links to specific data sets. For a complete list of CFG data and resources relating to this paradigm, see the CFG database search results for Siglec-4.

Glycan profiling

Glycogene microarray

Probes for mouse and human MAG (under the name Siglec-4) have been included on all four versions of the CFG glycogene microarray.

Knockout mouse lines

Glycan array

Related GBPs

Compared to other Siglecs, Siglec-4 is most conserved. Based on sequence similarity orthologous proteins can be identified in all vertebrate genomes available so far (several mammals, chicken, Xenopus, zebrafish and fugu). Sialic acid binding activity selective for 2,3-linked Sia has been shown for the avian ortholog (SMP[12] and fish Siglec-4 from zebrafish (Danio rerio) and fugu (Takifugu rubripes)[23]). Whereas the primary sequences of the Sia-binding N-terminal domains is 97 % identical between rodents and man and share over 50 % sequence identity between fish and mammals, the cytoplasmic tail is much less conserved (20% identical amino acids between fish and mammals[23]).